This invention is generally directed to liquid developer compositions and, more specifically, the present invention relates to processes for the preparation of liquid developers. More specifically, the present invention relates to a continuous process for the preparation of liquid developers comprised of liquid carrier, charge director, resin, carrier liquid, colorant, especially pigment, charge control agent, and optionally charge director and charge adjuvant, and wherein grinding is avoided thereby eliminating or minimizing contamination of the resulting developer by, for example, grinding media such as steel balls. In embodiments, the process of the present invention comprises the generation of a dispersion by extrusion of pigment and charge control agent in a toner resin, such as a thermoplastic resin, and subsequently cooling the molten mixture resulting under high shear to enable fine particles in a carrier fluid. Advantages associated with the economical processes of the present invention include permitting the direct preparation of high solids contents liquid developers, for example about at least 35 percent of solids and a reduction in process time. The developers of the present invention can be selected for a number of known imaging systems, such as xerographic imaging and printing processes, wherein latent images are rendered visible with the liquid developers illustrated herein. Charge control agents are considered soluble in the carrier liquid and they can charge the toner directly; charge adjuvants are soluble in the carrier liquid and increase the charge on the toner; and charge control additives are bound to the toner particle during use and can increase the charge on the toner.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. The dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to an intermediate, or a receiver sheet.
Typical liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m (microns) average by area size as measured with the Malvern 3600E particle sizer.
Since the formation of proper images depends primarily on the difference of the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it is desirable to add a charge director compound and optional charge control agents, or charge adjuvants which increase the magnitude of the charge, such directors being, for example, polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, metallic soaps, and the like, to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant.
Processes for the preparation of liquid electrostatic developer of the present invention include, for example, mixing, in a nonpolar liquid with the thermoplastic resin, charge control agent, and colorant like pigment in a manner that the resulting mixture contains about 15 to about 30 percent by weight of solids; heating the mixture to a temperature of from about 70.degree. C. to about 130.degree. C. until a uniform dispersion is formed; adding an additional amount of nonpolar liquid sufficient to decrease the total solids concentration of the developer to about 10 to about 20 percent by weight; cooling the dispersion to about 10.degree. C. to about 50.degree. C.; adding a charge director compound to the dispersion; and diluting the dispersion to 1 percent to 2 percent solids; and wherein the resin, pigment and charge control agent are dispersed in the liquid and charge director mixture.
In the initial mixture, the resin, colorant and charge control agent may be added separately to an appropriate vessel which can vary in size from 50 milliliters to 1,000 liters, such as, for example, an attritor, heated ball mill, heated vibratory mill, such as a Sweco Mill (manufactured by Sweco Company, Los Angeles, Calif.) equipped with particulate media for dispersing and grinding, a Ross double planetary mixer (manufactured by Charles Ross and Son, Hauppauge, N.Y.), or a two roll heated mill, which requires no particulate media. Useful particulate media include materials like a spherical cylinder selected from the group consisting of stainless steel, carbon steel, alumina, ceramic, zirconia, silica and sillimanite. Carbon steel particulate media are particularly useful when colorants other than black are used. A typical diameter range for the particulate media is in the range of 0.04 to 0.5 inch (approximately 1.0 to approximately 13 millimeters).
Sufficient nonpolar liquid is added to provide a dispersion of from about 15 to about 50 percent solids. This mixture is then subjected to elevated temperatures during the initial mixing procedure to plasticize and soften the resin. The mixture is sufficiently heated to provide a uniform dispersion of all the solid materials of, for example, colorant, charge control agent, and resin. However, the temperature at which this step is undertaken should not be so high as to degrade the nonpolar liquid or decompose the resin or colorant if present. Accordingly, the mixture in embodiments is heated to a temperature of from about 70.degree. C. to about 130.degree. C., and preferably from about 75.degree. C. to about 110.degree. C. The mixture may be ground in a heated ball mill or heated attritor at this temperature for about 15 minutes to 5 hours, and preferably about 60 to about 180 minutes. After grinding at the above temperatures, an additional amount of nonpolar liquid may be added to the dispersion. The amount of nonpolar liquid to be added at this point should be an amount sufficient to decrease the total solids concentration of the dispersion to about 10 to about 20 percent by weight.
The dispersion is then cooled to about 10.degree. C. to about 50.degree. C., and preferably to about 15.degree. C. to about 30.degree. C., while mixing is continued until the resin admixture solidifies or hardens. Upon cooling, the resin admixture precipitates out of the dispersant liquid. Cooling is accomplished by methods such as the use of a cooling fluid like water, glycols, such as ethylene glycol, in a jacket surrounding the mixing vessel. Cooling is accomplished, for example, in the same vessel, such as an attritor, while simultaneously grinding with particulate media to prevent the formation of a gel or solid mass; without stirring to form a gel or solid mass, followed by shredding the gel or solid mass and grinding by means of particulate media; or with stirring to form a viscous mixture and grinding by means of particulate media. The resin precipitate is cold ground for about 1 to 36 hours, and preferably from about 2 to about 6 hours. Additional liquid may be added at any time during the preparation of the liquid developer to facilitate grinding or to dilute the developer to the appropriate percent solids needed for developing. Other processes of preparation are generally illustrated in U.S. Pat. Nos. 4,760,009; 5,017,451; 4,923,778 and 4,783,389, the disclosures of which are totally incorporated herein by reference.
There is illustrated in U.S. Pat. No. 4,923,778, a process for the preparation of high solids liquid developers wherein grinding media are selected and thus contiminate the resulting developer, a problem avoided with the processes of the present invention. The aforementioned contimination adversely affects the developer characteristics obtained including, for example, causing arcing between the image bearing member surface and the reverse roller, causing scratching and premature wear of the image bearing member and other wetted rotating components present in the liquid development apparatus.
In U.S. Pat. No. 5,035,972, the disclosure of which is totally incorporated herein by reference, there are illustrated liquid developers with quaternized ammonium AB diblock copolymer charge directors, and wherein the nitrogen in the ionic A block is quaternized with an alkylating agent and processes thereof.
U.S. Pat. No. 5,019,477, the disclosure of which is hereby totally incorporated by reference, illustrates a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors, such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthenates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer (iii) selected from the group consisting of vinyl toluene and styrene, and (iv) selected from the group consisting of butadiene and acrylate. A copolymer of polyethylene and methacrylic acid or methacrylic acid alkyl esters, NUCREL.RTM., may also be selected.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. These liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
In U.S. Pat. No. 4,707,429, there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with certain charge directors are illustrated in U.S. Pat. No. 5,045,425.
In U.S. Pat. No. 5,306,591 and U.S. Pat. No. 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid, respectively and processes thereof. In U.S. Statutory Invention Registration No. H1483, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, and a charge director comprised of an ammonium AB diblock copolymer of the formula ##STR1## wherein X-- is a conjugate base or anion of a strong acid; R is hydrogen or alkyl; R' is alkyl; R" is an alkyl group containing from about 6 to about 20 carbon atoms; and y and x represent the number average degree of polymerization (DP) wherein the ratio of y to x is in the range of from about 10 to 2 to about 100 to 20.